Thermoforming apparatus with improved press

ABSTRACT

A thermoformer apparatus having a press driving upper and lower platens by respective pairs of servo motors, each pair driving four toggle linkages to stabilize the platen against the forming pressures. Linear bearings are provided on four corner posts for precision guiding of the platen motion.

BACKGROUND OF THE INVENTION

This invention concerns thermoforming apparatus, and more particularlypresses used in thermoforming parts from preheated plastic sheetmaterial. Thermoforming is in wide spread use as a manufacturing method,and involves preheating of thin plastic sheet material in an oven andthen advancing the same into a thermoforming press having a mold locatedbetween the press platens.

The plastic sheet is drawn against the mold surfaces during the formingoperation carried out by the application of a vacuum or air pressurecausing the sheet material to be drawn against the mold surfaces.Relatively large articles are often manufactured this way andconsiderable press loads are generated by the fluid pressure forcesacting over large areas, which must be resisted by the press platens andtheir supporting structure.

The forces required also may be increased by the use of plug assisttools in which complementary tooling is carried by one of the platensand driven into the forming surfaces on the other tooling piece. Theweight of the tooling supported by the platens thus can becomeconsiderable. A large servo motor has sometimes been used, the size ofthe motor in turn resulting in a relatively slow response time of theplaten drive mechanism. The relatively thin wall thickness of the partmakes accuracy in the forming process more critical.

Presses used in thermoforming typically involve a pair of columnssupporting the platens in their up and down motion and plain bearingsare used in the linkages used to drive the platens up and down and toguide the platens on the machine frame. This construction haslimitations in that the deflections of the platens under heavy loadingis not sufficient to prevent inaccuracies in the thin walled partsformed due to slight tilting and bending of the platens and thestructure supporting the platens under the heavy press loads described.In addition, a single large servo motor has considerable inertiaincreasing the cycle time required. This construction also absorbs asignificant portion of the motor power to overcome the frictionallosses. Where the apparatus is taxed to its utmost capability, theselosses become critical, and the press may not be able to execute theoperation required.

Accordingly, it is an object of the present invention to provide athermoforming apparatus having an improved thermoforming press in whichthe stability and precision movement of the platens is substantiallyimproved over the prior art thermoforming presses.

It is a further object of the present invention to provide a press foruse in thermoforming operations in which the frictional losses aresignificantly reduced.

It is yet another object of the present invention to provide a servomotor operated platen mechanism in which the response capability of theservo motor drive is improved over the prior art designs for heavierduty presses.

SUMMARY OF THE INVENTION

These objects and others which will become apparent upon a reading ofthe following specification and claims are achieved by a thermoformingpress in which the platens are guided on precision antifriction bearingslocated at each of four corners of each of upper and lower pressplatens, the bearings mounted on each of four posts forming a part ofthe press frame. Both the upper and lower platens are thereby guided oneach of their four corners with minimal friction losses, highlyprecision guided movement of the platens is obtained, and the platen isvery stably supported by the linear antifriction bearings at each cornerpost.

Actuation of each platen is carried out by a set of four separatelinkages connected to each plate at a location adjacent the four cornersof the plated, such as to evenly distribute the forces imposed on theplaten by the press forces.

Two separate servo motors are provided for each platen drive, each servomotor driving a transmission having an output shaft having a pair ofcrank arms affixed thereto, in turn driving two of the four togglelinkages provided for each platen.

Both the upper and lower platen are mounted and driven in the samemanner, such that a total of four servo motors driving two sets of fourtoggle linkages is employed for operation of the platens.

The pivotal connections of the links making up the toggle linkages areprovided with antifriction bearings at the pivotal connections such asto further reduce the frictional losses in the system.

The dual servo motor drive for each platen provides a rapid responsepress operation. A very stable support of the platens, even under heavyloading is achieved, and an only minimal frictional losses areexperienced such as to maximize the capacity of the press for conductingthermoforming operations.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view of a thermoforming press according to thepresent invention.

FIG. 2 is a front elevational view of the press thermoformer press shownin FIG. 1 with a block diagram representation of other components of thethermoformer apparatus.

FIG. 3 is a side elevational view in partial section along the lines 3—3in FIG. 2 with a block diagram representation of other components of thethermoformer apparatus.

FIG. 4 is an enlarged view of the section 4—4 taken in FIG. 2.

FIG. 5 is an enlarged fragmentary view of a portion of the thermoformerpress shown in FIG. 1 showing details of the shut height adjustmentmechanism.

DETAILED DESCRIPTION

In the following detailed description, certain specific terminology willbe employed for the sake of clarity and a particular embodimentdescribed in accordance with the requirements of 35 USC 112, but it isto be understood that the same is not intended to be limiting and shouldnot be so construed inasmuch as the invention is capable of taking manyforms and variations within the scope of the appended claims.

Referring to FIG. 1, a press 10 according to the present inventionincludes a press frame 12 formed by a top 14 and a bottom 16 joined byfour corner posts 18 defining a rectangular in section cavity withinwhich an upper platen 20 and a lower platen 22 are mounted to be capableof vertical motion towards and away from each other.

A pair of side plates 24, 26 overlie the outer sides of each of therespective pairs of corner posts 18, secured with machine screws to theouter sides of the respective pairs of corner posts 18 to stiffen theframe and provide outriggers for antitip leveling feet 26 located to thefront and rear of the press 10.

According to one aspect of the present invention, each corner of theupper and lower platens 20, 22 is supported by a precision linearantifriction bearing sets 28 and 30 each affixed to an inwardly facingsurface of each frame post 18, with a suitable bearing slide affixed toeach corner of the upper platen 20 and lower platen 22. This providesprecision low friction guidance of each platen in its up and down motionwithin the frame 12. The platens 20, 22 may be temporarily held inposition by means of a bracket 34 affixed thereto and a drilled plate 36on one of the posts 18. The bottom platen 22 may also be held inposition by means of a drilled plate 36 and bracket 38 such as to allowthe platens 20, 22 to be held stationary for maintenance purposes.

According to the concept of the present invention, each of the upper andlower platens 20, 22 is driven by a drive arrangement comprised a pairof electric servo motors 40A and 40B for the upper platen and a pair ofservo motors 42A, 42B for the lower platen 22.

Each servo motor 40A, 40B, 42A, 42B, drives a respective one oftransmission units 44A, 44B, 46A, 46B. The motor-transmission assembliesthereby formed are each held on one of the side plates 24 by means of anadapter 48. An output shaft 50A, 50B extending from each of thetransmissions 44A, 44B oscillates a main crank arm 52A, 52B,respectively, as well as auxiliary crank arms 54A, 54B.

A first connecting bar 56A drivingly connects auxiliary crank arms 54A,54B together to oscillate insuring simultaneous oscillation of both ofthe output shafts 50A, 50B of the transmissions 44A, 44B. This stallsboth motors 40A, 40B if one stalls to prevent one motor from continuingto operate if the other stalls.

The main crank arms 52A, 52B are each journaled to one end of one of apair of doubled linkage members 58A, 58B. The pivoting thereon ispreferably provided by low friction ball or roller bearings 60A, 60B,respectively, in order to reduce the frictional losses in the system.

The lower end of each double link 58A, 58B is journaled to a respectivebearing block 62A, 62B again by means of low friction rotary bearings64A, 64B. The bearing blocks 62A, 62B are secured to the top of theupper platen 20 by a shut height adjustment means to be describedhereinafter. The transmission shafts 50A, 50B extend across the frontalwidth of the machine 10 and over a second pair of double links 66A, 66B.Main crank arms 68A, 68B are journaled to the upper end of the doublelength links 66A, 66B, while the lower end of the double links 66A, 66Bare journaled in bearing blocks 70A, 70B fixed to the top of the upperplaten 20.

A second connecting bar (not shown) is operated by a pair of auxiliarycrank arms 72A, 72B also mounted to output shafts 50A, 50B.

Lower transmissions 46A, 46B likewise have respective output shafts 74A,74B which each drive a main crank arm 76A, 76B which are each pivotallyconnected to the lower end of a double links 80A, 80B on the right handside of the machine as viewed in FIG. 2, with the upper end of each ofthe double links 80A, 80B pivotally mounted in bearing blocks 82A, 82Bfixed to the undersurface of the lower platen 22.

A second pair of double links 84A, 84B are also driven by the shafts74A, 74B by means of main crank arms 86A, 86B pivotally connectedthereto with the upper ends of the linkages 84A, 84B, likewise journaledin bearing blocks 88A, 88B affixed to the undersurface of the lowerplaten 22. Auxiliary cranks 90A, 90B, 91A, 91B are provided, driven bythe shafts 74A, 74B, respectively, to drive connecting bars 92A, 92B forthe purpose described above.

The transmission output shafts 50A, 50B, 74A, 74B are each oscillated bythe connected servo motor, to produce a dead center condition shown inthe drawings at one extreme position, which is the bottom dead centerposition of the toggle linkage formed by the main crank arms and doublelinkage, a full stroke of the platens 20, 22. Return oscillation of thecrank arms, 52A, 52B, 76A, 76B draws each of the upper platen 20 andlower platen 22 apart from each other.

The combination of the crank arm 62A and the double links 58A, 58B, 64A,64B, 66A, 66B, 86A, 86B and links 84A, 84B act as toggle linkages ableto exert a powerful mechanical advantage. In the aligned, dead centerposition shown, the platens 20, 22 are able to be held in positionagainst the enormous thermoforming pressures typically exerted. The fourpoint location of the four links mounted to each platen with a doublelink adjacent each corner of a platen, together with the location of theprecision bearings 28, 30, provide a very precise guided platenmovement, and stabilized support for the platens 20, 22 despite theheavy forces imposed, such that very good accuracy of the thermoformedparts may be achieved.

An auxiliary lifting cylinder 96 for the upper platen and auxiliarylifting cylinder 78 for the lower platen is provided to assist inovercoming the weight of the tooling dies, shown diagrammatically inphantom lines in the drawings. Such assist cylinders are well known andtypically comprise pneumatically actuated devices.

The press shut height may be adjusted by means of slide wedges 100A,100B, 102A, 102B interposed beneath the bearing blocks 62A, 62B, 70A,70B.

An adjustment motor 104 is operatively connected to cause slidingmovement of the wedges 100A, 100B and via a cross shaft 106, also thewedges 102A, 102B by means of a power screw device of a type well knownin the art and hence will not described herein in detail.

What is claimed is:
 1. A thermoforming apparatus including a press, apreheat oven preheating plastic sheet material to be formed into shapedarticles in said press, means for feeding sheet material through saidoven and into said press, means for creating a differential pressureacting on said sheet material to force said sheet material againsttooling in said press to be formed, said press including; a machineframe; an upper platen; a lower platen; said machine frame including atop structure and a bottom structure connected together by four cornerposts extending therebetween, said upper and lower platens confinedwithin said corner posts for movement towards and away from each otherto bring thermoforming tooling together to form an article and toseparate said thermoforming tooling to allow loading of sheet materialinto said tooling and to remove a formed article: said upper and lowerplaten each guided by linear bearing means on an inside surface of eachof said four corner posts and outside surfaces of sides of said upperand lower platens to guide movement thereon during said platen movementtowards and away from each other; an upper platen drive means includinga set of four toggle linkages drivingly connected to an upper side ofsaid upper platen in a rectangular pattern to establish four points ofsupport for said upper platen to resist forces generated bythermoforming of said articles; said upper platen drive means furtherincluding motor means driving said set of linkages drivingly connectedto said upper platen to cause movement of said upper platen towards andfrom said lower platen; lower platen drive means including a set of fourtoggle linkages drivingly connected to a lower side of said lower platenin a rectangular pattern to establish four points of support for saidlower platen to resist forces generated by thermoforming of saidarticles; said lower platen drive means further including lower platenmotor means driving said set of four toggle linkages connected to saidlower platen to cause said movement of said lower platen toward, andaway from said upper platen; said upper platen motor means including twoseparate servo drive motors each having an output shaft drivinglyconnected to respective pairs of said four linkages connected to saidupper platen, and said lower platen motor means including two separateservo motors each having an output shaft drivingly connected torespective pairs of said four linkages connected to said lower platen;and further including a positive driving interconnection between saidoutput shafts of said two servo motors of each of said upper and lowerplaten drive means positively constraining simultaneous rotation of saidoutput shafts of said upper platen and lower platen drive means, wherebyif one servo motor stalls, the other servo motor in said respectiveupper or lower platen drive is stopped.
 2. The thermoforming apparatusaccording to claim 1 wherein each servo motor of said upper and lowerplaten drive means includes output shafts extending across therespective upper and lower platens, wherein each set of said four togglelinkages includes a pair of spaced apart crank arms connected to eachoutput shaft, a separate link with each crank arm of said two servomotors and a pivotal connection between each crank arm and one end ofsaid link, an opposite end of each of said four links in each set oftoggle linkages pivotally mounted to said upper and lower platen,respectively, oscillation of said output shafts driven by respectiveservo motors causing oscillation of said links to cause movement of saidupper and lower platens, said crank arms each becoming aligned with arespective link in a fully extended position in which said upper andlower platens are drawn closest together.
 3. The thermoforming apparatusaccording to claim 2 wherein each pivotal connection of each of saidlinks includes an antifriction bearing.
 4. The thermoforming apparatusaccording to claim 2 wherein each servo motor and the output shaft areinterconnected by a transmission unit.